The present invention relates to determining state of charge of an electrochemical device. More particularly, the present invention relates to determining state of charge of an electrochemical device using an intelligent system, e.g., a fuzzy logic system.
Presently there are three basic approaches to determining the state of charge (SOC) of batteries, Coulomb counting, response of a battery to an applied load, and batter impedance measurements.
The Coulomb counting approach involves the measurement of charge flow out of the battery into the load and charge flow from a battery charger to the battery. The Cumulative charge removed from the battery is used to estimate the battery's SOC with or without corrections being made for changes in battery capacity with discharge rate and battery temperature. The corrections to battery capacity made to compensate for battery temperature and discharge rate variations are performed using semi-empirical equations, such as the Peukert equation, as demonstrated by U.S. Pat. No. 5,656,919. Other analytical equations used to adjust the battery SOC for discharge rate and battery temperature were reported by Alzieu et al in Journal of Power Sources 67 pp. 157-161 (1997). Another approach to coulomb counting is to use actual battery data stored in lookup tables to adjust for the battery SOC variation with discharge rate and battery temperature. A good example of this method is provided in U.S. Pat. No. 5,372,898. Another example of this approach was presented by Freeman and Heacock in the High Frequency Power Conversion 1994 Conference Proceedings pp. 231-241 (April 1994). These methods are commonly used in portable electronic applications but do not always work well in the case of variable loads. They are also not particularly accurate when interpolating between the data available in the lookup tables.
The response of a battery to a load can be used to determine battery SOC. By observing the voltage of the battery before, during, and after the application of a load, the battery response to the load may be used to determine the battery SOC. One example of this approach was presented by Peled in U.S. Pat. No. 4,275,784. This patent describes a means of inferring the SOC of lithium batteries by using the battery temperature and the temporal voltage recovery profile of a battery after the removal of a fixed load on a battery, by comparison with reference tables for the given temperature and voltage recovery characteristics. A similar approach has been reported by Finger in U.S. Pat. No. 4,460,870. These methods like the Coulomb counting methods above do not necessarily provide reliable interpolation between the conditions at which the batteries have been previously calibrated.
It is well known that the measurement of the internal impedance of a battery may be used to determine the battery's state of charge (SOC). U.S. Pat. No. 3,562,634 describes a technique where the capacitances of nickel-cadmium batteries are used to determine the SOC of these batteries. This approach, however, is awkward to implement because it requires a standard reference curve to be used for comparing the measured capacitance in order to infer the battery's SOC.
Several patents, including U.S. Pat. Nos. 4,678,998, 5,241,275, 5,650,937, and 5,369,364, and publications including Journal of Applied Electrochemistry, Blanchard, 22 pp. 1121-11128 (1992), Journal of Applied Electrochemistry, Viswanathan et al., 9 pp. 125-139 (1979), Journal of Applied Electrochemistry, Karunithalaka et al., 13 pp. 577-586 (1983), and Doctoral Dissertation of John Weckesser, entitled An Evaluation of the Electrochemical Properties of Metal Hydride Alloys For Rechargeable Battery Applications, Rutger University (1993) describe different extraction techniques for determining the SOC of a battery having measured the complex internal impedance of the battery at various frequencies. These methods are cumbersome, computationally intensive, and difficult to automate. Also, battery-to-battery variation can be large enough that these techniques become unreliable.
Another approach to determining battery SOC from battery impedance measurements is described in U.S. Pat. No. 3,984,762. This patent describes a means for measuring the phase angle of the complex impedance of a battery and a method for correlating this information with the battery's SOC. Although this method is relatively easy to implement, it is of limited value in that many batteries do not have a consistent correlation between the phase angle of their internal complex impedance and battery SOC.
U.S. Pat. No. 4,743,855 describes an analytical method for determining battery SOC simply by measuring the battery's complex internal impedance at only two frequencies. This considerably simplifies the required electronic circuitry and required computation compared to having to measure the battery's complex internal impedance at many frequencies. Although, this analytical method may work reasonably well for some primary batteries, it is not well suited for rechargeable batteries.
U.S. Pat. No. 5,132,626 a method is described whereby the internal impedance is measured at a single frequency and the battery SOC inferred from this measurement. The drawback of this approach, in general, is the difficulty in finding a single frequency at which the magnitude of the complex internal impedance of the battery correlates well with the battery's SOC.